Known electronic devices for safeguarding vehicle occupants, in particular motor-vehicle occupants, typically comprise acceleration-sensitive sensors, which register the accelerations acting on the vehicle and, when high acceleration values indicate danger, cause safety devices provided for vehicle occupants, such as belt tighteners and/or airbags, to be tripped in time. Dangerous, high acceleration values occur, in particular, when a vehicle experiences a collision or when it crashes against a stationary object.
An exceptionally high operational reliability is required of electronic devices of the above-mentioned type, which are critical to safety. In an emergency, these devices are entrusted with passengers' lives. This high degree of operational reliability can be virtually guaranteed only when all the components of the electronic device are able to be tested in at least regular intervals.
The acceleration-sensitive sensors, which are not able to be easily tested during the normal driving state of a vehicle without entailing considerable expenditure, present a special problem in this case. A principal reason for this is the fact that during normal vehicle driving states, the acceleration curve shapes that are characteristic of an accident do not occur as a function of time; nor do high amplitude values occur. It may be that, in principle, the acceleration-sensitive sensors can be tested when the vehicle is in a state of non-readiness. To this end, the acceleration-sensitive sensors must be disassembled, for example, and subjected to an acceleration test on a test stand. As an alternative, the vehicle could be exposed, with the acceleration-sensitive sensors installed, to realistic acceleration values in a simulated crash test. Even when such costly testing methods find the acceleration-sensitive sensors to be usable, there is no guarantee that the operability will be sustained over a long period during a normal driving state.
U.S. Pat. No. 4,873,452 describes a testable electronic device having a piezoelectric, acceleration-sensitive sensor. An electric testing pulse is applied to the sensor for diagnostic purposes. The piezoelectric sensor reacts to this electric testing pulse by emitting an output signal capable of being analyzed by an evaluating circuit. Apart from the fact that the additional testing device is very expensive and results in a very costly electronic device, it is not possible to test the sensor using this known testing method in a way that will correspond to actual service conditions. One may be able to test the electrical properties of the sensor; however, one cannot test to see if the sensor reacts in the desired manner to a mechanical action of force resulting from an applied acceleration. In the same way, the mechanical coupling of the acceleration-sensitive sensor to the vehicle cannot be tested.
An acceleration-sensitive sensor having a testing device is described in U.S. Pat. No. 4,950,915. The testing device comprises an acoustic source, which applies acoustic waves to the acceleration-sensitive sensor for testing purposes. The acoustic waves are injected into the acceleration-sensitive sensor, on the one hand, indirectly, on the other hand, directly through a housing surrounding the sensor and the acoustic source. In this manner, besides fundamentally testing the transducing properties of the acceleration-sensitive sensor, one is supposedly also able to test the mounting of the sensor in the vehicle. Apart from the fact that the additional acoustic source and its trigger circuit make the known electronic device more intricate and expensive, there is an elevated risk, due to the greater number of components, of errors also occurring in the testing device.
Furthermore, German Provisional Patent No. 24 54 424 discloses that in the case of one safety device for vehicle occupants, acceleration values that fall below a specifiable minimum value are not evaluated. Normal shaking of the vehicle due to road conditions, for example, also falls below this value. As a rule, this is justified by the fact that such low acceleration values do not present any danger to the vehicle occupants. Therefore, output signals from the acceleration-sensitive sensors, which can be attributed to such low acceleration values, are suppressed. German No. C2-26 12 215 describes, for example, that acceleration values of safety devices are only evaluated above a limiting value of 4 g. Accelerations below this value are disregarded.
Finally, German No. A1-39 20 091.4 describes a vehicle occupant safety device comprising two acceleration-sensitive sensors, which compares the sensor output signals for purposes of testing the operability of the sensors. The disclosed device makes use of the fact that high acceleration values occur temporarily in normal driving states as well. These values are not accident-specific, but rather can be attributed to the heavy stresses that the vehicle body and the sensors are subjected to, for example, when the vehicle is driven over potholes, irregular road surfaces or the like. However, such stresses due to acceleration occur quite irregularly, so that a reliable test cannot be set up on the basis of these stresses. Furthermore, the disclosed device presupposes the application of acceleration-sensitive sensors having largely conforming output-signal characteristics. However, this necessitates a complex selection process with a comparatively low yield of sensors having conforming output-signal characteristics. Sensors selected in this manner, however, are comparatively expensive.